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Genes linked to chronic conditions including Alzheimer’s may be gradually growing less common thanks to natural selection. Andrew Masterson reports.

Given enough time, chronic conditions such as obesity, high cholesterol, Alzheimer’s disease and even heavy smoking might disappear from humanity without the need for medical intervention or major lifestyle changes.

The only problem is, the process will take a large number of years – enough, in fact, for scores of generations to live, die and, critically, reproduce.

The researchers analysed genomes of more than 210,000 people in the US and UK. In doing so, they discovered that genes, or groups of genes, associated with some chronic conditions were less common in people who lived to older ages.

On a basic metric, people who live into old age are more likely to have successfully passed on their genes more often than people who die relatively young.

People who lacked gene-groups thought to be associated with the chronic conditions were better represented in the older cohort than those with the suspect genes. It is reasonable to assume that, over many generations, their descendants will eventually outnumber everyone else.

The other major pattern that was found concerned a gene called CHRNA3, which is linked to heavy smoking in men. The genomes revealed that the presence of the gene became less frequent in older people, starting in middle age.

“It may be that men who don’t carry these harmful mutations can have more children, or that men and women who live longer can help with their grandchildren, improving their chance of survival,” says coauthor Molly Przeworski.

Of course, many chronic conditions are influenced by hundreds of genes and mapping their effect is challenging. To moderate this phenomenon Przeworski and her colleagues looked at 42 common conditions in tandem with the genomes of each person in the study.

They found that a predisposition to high cholesterol or a BMI at obese level were associated with early deaths. Interestingly, they also found that, for women, delayed puberty and delayed age of first childbirth were both associated with small but significant increases in longevity.

The findings suggest several targets for research, but, the geneticists warn, genetic resilience (or vulnerability) is also heavily influenced by environmental factors.